A significant class of optical devices are commonly called “planar light-wave circuits” or “planar light-wave chips” or just PLCs. PLCs comprise technologies wherein optical components and networks are disposed monolithically within a stack or stacks of optical thin films supported by a common mechanical substrate such as a semiconductor or glass wafer. PLCs are typically designed to provide specific transport or routing functions for use within fiber-optic communications networks. These networks are distributed over a multitude of geographically dispersed terminations and commonly include transport between terminations via single-mode optical fiber. For a device in such a network to provide transparent management of the optical signals it must maintain the single-mode nature of the optical signal. As such, the PLCs are commonly, though not strictly, based on configurations of single-mode waveguides. Since optical signals do not require return paths, these waveguide configurations do not typically conform to the classic definition of “circuits”, but due to their physical and functional resemblance to electronic circuits, the waveguide systems are also often referred to as circuits.
The standard family of materials for PLCs, widely demonstrated to have superior loss characteristics, is based on silicon dioxide, commonly called silica. The silica stack includes layers that may be pure silica as well as layers that may be doped with other elements such as Boron, Phosphorous, Germanium, or other elements or materials. The doping permits control of index-of-refraction and other necessary physical properties of the layers. Silica, including doped silica, as well as a few less commonly used oxides of other elements, are commonly also referred to collectively as “oxides.” Furthermore, although technically the term “glass” refers to a state of matter that can be achieved by a broad spectrum of materials, it is common for “glass” to be taken to mean a clear, non crystalline material, typically SiO2 based. It is therefore also common to hear of oxide waveguides being referred to as “glass” waveguides. Subsequently, the moniker “silica” is used to refer to those silicon oxide materials suitable for making waveguides or other integrated photonic devices. It is important to note that in the context of this invention, other waveguide materials, such as lithium niobate, spin-on glasses, silicon, siliconoxynitride, silicon oxycarbide, polymers, group “III-V” materials such as InP, GaAs, and InGaAsP or other materials described in U.S. Pat. No. 6,614,977 (the entire content of which is hereby incorporated herein by reference), are also appropriate.
In a typical example of a PLC, a waveguide formed of a core material lies between a top cladding layer and a bottom cladding layer. In some instances, a top cladding may not be used. Waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. In this example, each layer is doped in a manner such that the waveguide has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the waveguide, the layers are typically situated on a silicon wafer. As a second example, waveguides can comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the waveguide may comprise an optically transparent polymer. Another example of a waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction.
Many integrated optical devices require the creation of physical structures that are highly symmetric. A key example is a planar waveguide coupler consisting of two optical waveguides coupled to each other across a gap. In many cases, the achievement of highest performance in such a coupler requires the two waveguides to be identical to each other. A conventional approach to achieving this goal is to define the optical waveguides using a photolithography and etching process with a pattern consisting of two waveguides of identical cross section separated by a gap. In this scheme, the fabrication of identical guides relies upon very high fidelity in the lithography and etching processes to reproduce the identical mask patterns into the optical waveguides. This strategy will allow integrated optical couplers to be fabricated with a certain level of performance that may be adequate for some types of devices. However, the ultimate performance may be limited due to asymmetries induced by the fabrication process.
One structure that may be used to evaluate the performance of coupler structures is a beat-length coupler. Such a coupler consists of two optical waveguides that are brought into close proximity to each other and optically couple such that at a certain wavelength and polarization the optical power should be completely transferred from one guide (“bar”) to the other (“cross”). The ratio of the optical power in the bar output waveguide to the power in the cross output waveguide can be defined as an “extinction ratio”. This may be expressed in log scale as ER=10×log(Pbar/Pcross) decibels (dB). A high quality beat-length coupler test structure will have an extinction ratio that is a negative number of large magnitude at its optimal wavelength and polarization (“high extinction ratio”). The performance of a beat-length coupler test structure provides an indication of the performance of other couplers that have the same design parameters except for length. We have found that using the straightforward conventional approach it is difficult to achieve consistent extinction ratios below −20 dB in high index contrast planar lightwave circuits (PLCs). A high index contrast planar lightwave circuit typically has a core to cladding refractive index greater than 0.03.
The extinction ratio of an integrated optical coupler, for example, can be degraded by asymmetries introduced in the fabrication process. The present invention greatly reduces a critical type of fabrication-induced asymmetry.
The outline of a process to produce PLCs of integrated optical waveguides is as follows:
a. Grow a higher index waveguide core layer on a lower index cladding layer on a carrier substrate wafer
b. Spin photoresist on the waveguide core layer, expose using a suitable pattern, develop the photoresist
c. Etch the waveguide core layer using the photoresist pattern as a resistant etch mask
d. Remove the remaining resist mask and deposit lower index cladding material on the wafer
The photolithography step is fundamental to creating a device with a physical pattern that closely resembles the intended design. Integrated optics technology makes use of a number of sophisticated technologies devised for the semiconductor industry. A key such technology is exposure of patterns using a projection stepper. A stepper illuminates a patterned photomask reticle with ultraviolet light and projects a focused image onto the photoresist layer surface using a precision multi-element lens system. The pattern of ultraviolet light intensity at the photoresist layer (“aerial image”) exposes the photoresist, causing chemical changes that will allow a detailed pattern remain on the wafer surface after a chemical development process. Ideally, if a symmetric pattern exists on the photomask reticle, it should produce a symmetric aerial image, and therefore a symmetric photoresist pattern. However, it is possible for the stepper imaging lens system to induce distortions that result in the production of an asymmetric aerial image from a symmetric pattern on the reticle.
It is well known that any real lens has imperfections that result from limitations in the lens design or residual imperfections from manufacture. For example, shown in FIG. 1 is a stepper distortion field vector map illustrating stepper lens distortion due to aberrations in an exemplary stepper imaging lens system.
One manifestation of these imperfections is a displacement of the location of a pattern on the wafer surface from its ideal intended location. It is possible to observe the variation of these displacements across the field of the lens by using a special test reticle and position measuring apparatus on the interferometer controlled motion stage of the stepper. An example of such a distortion map is included here as FIG. 1. These distortions are caused by aberrations of the lens. The manufacturer of the stepper lens has attempted to correct these aberrations to a great degree through design and control of the shapes and compositions of the lens elements, but there are always some uncorrected aberrations left in each lens. The effects of these residual aberrations can vary across the field of the lens.
An example of a well-known type of lens aberration is coma. In coma, optical rays away from the axis of an imaging system are improperly focused and shifted from their desired focal position. Although primary coma aberration is typically targeted for correction by a stepper lens maker, it is likely that some residual coma-like aberrations remain. Although the primary effect of coma is to cause an image shift, it has been shown that it can also cause a localized asymmetry in an aerial and resist image (T. A. Brunner, IBM J. of Research and Development, Vol. 41 No. 1/2, January/March 1997, pp. 57-67, see page 62, http://www.research.ibm.com/journal/rd/411/brunner.pdf). We have simulated the projection of a conventional coupler image using typical parameters for our stepper. Simulation shows that moderate degrees of coma can create aerial images of waveguide couplers that have asymmetries of the order of 13 nm. Optical device simulation (figure included) shows that for our high index contrast couplers the beat-length coupler extinction ratio would be limited to −20 dB. This is in close agreement with results observed using couplers made without the benefit of the present invention. It should be noted that as the uncorrected aberrations vary across the lens field, the asymmetries induced in couplers due to this effect are expected to vary across the field. This would cause coupler extinction to vary depending on the location of a coupler on a photomask reticle. We have observed this effect experimentally.
In addition, asymmetry may be due to other steps in the process of fabricating a planar lightwave circuit, such as the step of etching. Etching is a chemical process that can be quite pattern dependent. For example, in a coupled waveguide system, the etching rate in the gap side of the waveguide is different than the etching on the outside of the waveguide. This leads to waveguides that have a “profile asymmetry”. Profile asymmetry is where a waveguide is not perfectly symmetric like a rectangle or a trapezoid. One side of the waveguide may be perfectly vertical, while the other side may be tilted. It is known that such profile asymmetry causes degradation of couplers.
Therefore, there is a need for a device and method to enhance the symmetry of waveguides used in PLCs of integrated optical devices. It is to such devices and methods that the present invention is directed.